10 must know Linux interview questions and answers

In order to point out the differences between the UNIX and Linux operating systems, you have to realize that Linux is a UNIX clone. There are several differences between the Linux and UNIX operating systems, including the following:

• A basic difference between UNIX and Linux is that Linux is an open-source operating system. As an open-source operating system, Linux implies that the source code is available for developers to adapt as needed. However, the UNIX operating system does not fall under the category of an open-source operating system that developers can modify.

• The fact that the Linux operating system is free is one of the main reasons for its widespread adoption. The Linux operating system is free to download on the internet, but not the UNIX operating system.

• When we compare how flexible and compatible both operating systems are, we know that Linux is more adaptable and more compatible with many types of hardware.

The acronym DOS stands for Disk Operating System, while BASH stands for Bourne Again Shell. A shell is just a user interface for interacting with an operating system's services. It can employ a command-line interface (CLI), a graphical user interface (GUI), or both. The following are the key differences between DOS and BASH:

• BASH commands are case sensitive, although DOS commands are not. In BASH, the characters / and - serve as a directory separator and an escape character, respectively. In DOS, the / character is used as a command parameter delimiter as well as a directory separator.

• DOS has a naming system in which a file must have an 8-character filename followed by a dot and a three-character extension. BASH does not have a name convention.

Under Linux, there are three types of permissions:

• Read: Users have the ability to read files or browse the directory.
• Write: Users have the ability to write to the file or create new files in the directory.
• Execute: Users can launch the file or search for a specific file within a directory with this option.

The chmod command can be used by a system administrator or the owner of a file or directory to grant permission to others. The receiver(s) of the permission is next specified, followed by whether the permission is granted or refused, the kind of permission, and lastly the file name.

To answer your question, the command, to provide permission to read and execute TuringDemo.TXT to the user and others, is:

chmod uo+rx TuringDemo.TXT

The GNU project was started with the goal of creating a free functioning framework for clients. Clients would be able to test, share, circulate, study, update, and improve the product, as well as bring out new features.

The goal of this project was to provide a free functional framework that included "everything valuable that generally accompanies a UNIX framework so that one could get by with no product that wasn't free."

Every other component of the Linux framework, with the exception of the piece, is GNU. It was taken under GNU variation 2 and the name Linux was changed to GNU/Linux as a result.

Linux allows a maximum length of 255 characters for any filename, excluding the pathname. This furthest point does not include the pathname, therefore the total pathname and filename length might easily exceed 255 characters.

Make sure that you mention the exclusion of pathname, as in a Linux interview, the interviewer would definitely ask you about the pathname.

These are the six stages of Linux boot process:

• BIOS: BIOS or Basic input/output system examines the integrity of the system and looks for the boot loader program on the CD-ROM or hard drive to load and run it. The BIOS grants control to the boot loader program after it has been discovered and loaded into memory. So, in a nutshell, the BIOS loads and executes the MBR bootloader.

• MBR: Master Boot Record (MBR) may be found on the bootable disk's first sector. Usually found in dev/hda or dev/sda. It contains GRUB-related information (Grand Unified Bootloader). So, in a nutshell, the MBR loads and runs the GRUB boot loader.

• GRUB: The default kernel image given in the grub configuration file is loaded by GRUB. /boot/grub/grub.conf is the location of the grub configuration file. It consists mostly of the kernel and the initrd image (initial ram disc - a method of loading a temporary root file system into memory). In a nutshell, GRUB loads and runs kernel and initrd images.

• Kernel: The root file system is mounted, and the /sbin/init program is run. The process id of init is 1 since it is the first program to be executed by the kernel. To double-check, run ps -ef | grep init.

• Init: Init determines the Linux run level by looking at the /etc/inittab file. In Linux, the following run levels are available:

halt

Single-user mode

Multi-user mode

Full Multi-user mode

Unused

X11

reboot

• Runlevel: Various services may be seen launching up as the Linux system is powering up. These are the runlevel programs, which are executed from the run level directory according to the run level's specifications.

In such Linux interview questions, make sure you talk about how you would approach and solve the problem. If you have any prior experience in solving such issues yourself, that would be a valuable addition in answering this Linux interview question.

/etc/passwd is a text file that Linux uses to store user account information. A one-way encrypted password is also stored in this file. It is accessible by numerous programs to obtain user information, posing a security concern; hence, the file must be 'Word Readable.' We may utilize the shadow password format to reduce the security risk. This approach stores account information in the /etc/passwd file on a regular basis.

The password, on the other hand, is saved as a single "x" character (not actually stored in this file). A second file, "/etc/shadow," stores encrypted passwords as well as other data such as account or password expiration values, and so on. Because the /etc/shadow file is only accessible by the root account, it poses less of a security concern.

Using ssh to connect to servers is extremely frequent. As a result, you can take the following measures to protect the SSH service:

• Disabling root login, as well as password-based logins, will improve the server's security.
• Disabling password-based logins and permitting key-based logins, which are safe but may be further restricted by only allowing access from specific IP addresses.
• Changing the default port to anything else reduces the number of random brute force attacks from the internet.
• By requiring the service to use only version 2 of the protocol, security and functionality enhancements will be introduced.
• The whitelist strategy may be used, where only people on a certain list are allowed to connect to the server through SSH.

Run the following command in the shell to see if the user account is locked:

passwd –S <username>

Another way is to look for the grep username in the /etc/shadow file, which will add a symbol '!' to the encrypted field in the password box.

Type the following command to simply unlock the password:

passwd –u <username>

Run the following command twice, if there is a double exclamation mark:

usermod –U <username>

To find out how much memory Linux is using, use the "Concatenate" command in the Linux shell.

When you use this command, you'll get a list of memory usages by Linux, including Total Memory, Free Memory, Cache Memory, and many others. Other Linux commands include:

• $ free –m // this is the most basic command that displays memory use in megabytes
• $ vmstat –s // this command generates a virtual memory statistics report
• top // this command examines memory and CPU use
• htop // this command is very similar to the top command